1. Field of the Invention
This invention relates to an optical disk for use in optical memory devices that conduct a recording operation, a regenerating operation, or an erasing operation of information using laser beams, and to a method for the production of the optical disk.
2. Description of the Prior Art
In recent years, optical memory devices have become recognized as a dense and mass memory device. Optical memory devices can be classified into three groups consisting of regenerative memory devices, write once memory devices and rewritable memory devices. Optical memory devices, which can be classified as write once memory devices and rewritable memory devices, generally have guide tracks on a disk made of glass or plastics so as to guide a light beam for recording and/or regenerating information to a given portion of the optical memory device. A portion of each of the guide tracks is formed into a pit-shape, resulting in a track address by which the position of the guide track can be identified. When the sectionalization of each guide track is needed to administer information, sector addresses are also disposed in the disk.
FIG. 10 shows a conventional disk 500 with guide tracks that are constituted by grooves 100. Information such as track addresses, sector addresses or the like disposed on the disk 500 in advance are formed into the shape of pits 200. In general, on a disk with guide grooves and pits, a recording medium is formed by the vacuum evaporation method, the sputtering method, the spin-coating method or the like. When necessary, a protective substrate and/or a resin film is laminated on the recording medium, resulting in an optical memory device.
Because information is recorded on the guide tracks 100 (FIG. 10) using light such as laser beams or the like, the shape of the grooves 100 significantly influences the tracking servo signal characteristics essential to keeping a light beam spot on a given guide track. To obtain good tracking servo signal characteristics, the depth of guide grooves formed in the disk are usually set to be around .lambda./8 n (wherein .lambda. is a wavelength of light and n is the refractive index of the disk). In contrast, because information such as track addresses, sector addresses or the like are formed into a pit-shape on the disk and read off by utilizing a diffraction effect of light in the pits, the depth of each pit is set to be around .lambda./4 n. In this way, the depth of guide grooves 100 of a disk 500 for use in optical memory devices is different from the depth of the pits 200 of the said disk. This kind of disk 500 is designed such that, as shown in FIG. 10, one surface 300 of the disk 500 is flush with the surface of the land 400 positioned between the adjacent guide grooves 100 that are formed on the said surface 300 of the disk 500.
FIGS. 11a to 11d show a process for producing the above-mentioned disk 500. As shown in FIG. 11a, a resist film 600, formed on a disk plate 10 is illuminated with a laser beam 700, resulting in guide grooves and pit latent images on the resist film 600. The intensity of illumination of laser beam 700 for formation of guide groove latent images is set lower than that of the formation of pit latent images. Then, the resist film 600 is developed, as shown in FIG. 11b, resulting in a pattern 660 corresponding to the guide grooves 100 and pits 200 shown in FIG. 10, the depths of which are different from each other. The disk plate 10 with a pattern 660 is then subjected to a dry or wet etching treatment, resulting in a disk 500 such as that of FIG. 10. Alternatively, as shown in FIG. 11c, a metal film 800 made of nickel (Ni) or the like is formed on the disk plate 10 with a pattern 660 by the sputtering method, the vacuum evaporation method and/or the electroforming method, resulting in a stamper (i.e., a master plate) 880 as shown in FIG. 11d. By the use of the stamper 880, a plastic disk with a structure such as that shown in FIG. 10 can be formed by an injection technique.
The shape of the bottom of each of the grooves 100 formed on a disk 500 is a transcription of the shape of the top surface of the patterned photoresist film 660. Thus the bottom face of each of the grooves of the disk 500 mirrors the unevenness of the top surface of the patterned resist film 660. This unevenness causes noise when information written into the guide grooves (i.e., guide tracks) 100 of the disk 500 using a laser beam is regenerated by a laser beam producing inferior regenerated-signals.